Rotary positive displacement pumps are based on the concept a rotating element that mechanically transports a volume of medium from a suction (inlet) end of the pump to the discharge (outlet) end during a revolution. A single revolution displaces a fixed volume of liquid, regardless of its viscosity. Typical examples of positive displacement pumps are internal gear pumps, external gear pumps, and rotary vane pumps. When a positive displacement pump is magnetically driven, the rotating elements can be incorporated into a hermetically sealed chamber, to avoid leakage of the pump medium during normal operation and in the event of pump failure. The combination of magnetic coupling and positive displacement suit the use of such pumps for pumping liquids, such as toxic media, chemicals, inflammable liquids, etc. where it is required or desirable to have a completely leak proof pump.
Partly due to eddy currents in the magnetic coupling caused by the rotation of the permanent magnet and partly because of bearing and hydraulic losses the magnetic coupling may get inadmissibly hot, so that cooling becomes necessary. This is obtained in known constructions by using the pressure drop across the pump to conduct a part of the pump medium through the coupling and bearings. This entails some disadvantages, however, in particular because the viscosities of the pump media are in themselves different, they are temperature dependent, and furthermore the pressure drop across the pump varies, so that there is no control of the rate of flow for cooling. This means that the cooling of the magnetic coupling and bearings has to be individually adapted to the specific pump medium and its temperature. The leakage of pump medium for cooling purposes means a reduced pump capacity.
The purpose of the invention is to provide an efficient cooling of the magnetic coupling and bearings of a positive displacement pump. Furthermore it is a purpose of the cooling system that there is no excessive internal leakage in the pump.
If the differential pressure of the pump is used to drive the cooling flow through the magnetic coupling, a problem occurs when pumping a high viscous medium while the pressure difference between suction and discharge port of the pump is relatively low. In this situation the flow through the coupling might become too small to guarantee the proper cooling of the coupling. Furthermore, if the viscosity of the pumped medium changes during pumping, the flow across the coupling would change as well, meaning cycles of excessive or insufficient cooling would be set up within the pump.
An aim present invention is to provide an improved internal gear pump having a cooling system that overcomes overheating experienced in the art.